Products wrapping systems can comprise a packing machine used particularly for groups of products in single packs using a strip of shrink-wrap material using where the said machines are of the type with preferably coplanar closed-ring horizontal conveyors and between which, according to Italian Patent No. 1,186.550, the stacking unit is inserted. A system of this type can comprise a first closed-ring horizontal conveyor coplanar with those of the machines as well as a pair of closed-ring longitudinal conveyors side by side, parallel and external on either of the said first horizontal conveyor and in which each conveyor of the pair of conveyors comprises one or more pairs of closed-ring conveyor devices side by side with the top branch of each pair of conveyor devices at different levels on respectively higher planes superimposed in relation to the plane of the first horizontal conveyor. The conveyor devices of the pair of conveyors are connected in an equidistant longitudinal position to a plurality of mirror symmetrical lifting devices for each pair of the conveyor devices. Operating devices serve to operate the said conveyor devices with continuous motion, to keep the mirror symmetrical lifting devices in a constant vertical attitude and to make the lifting devices oscillate around a horizontal axis parallel to the direction of movement of the respective conveyor devices to pick up and release the single packs being stacked.
These units are designed to stack packs particularly in wrapping systems that use a strip of shrink-wrap material in by using chains. Although they give excellent results as regards the quality of the stacked products and productivity per unit of time, i.e. they are high-speed units, the apparatus is referred to in the relevant technological jargon as operationally "rigid" units precisely because the corresponding conveyors are in the form of chains which have reached production speed limits which cannot be exceeded, i.e. a maximum speed which in the specific production sector is defined as second speed and so, obviously, these units are regarded as second-speed, or speed "two" units. These units can be contrasted with other units having chains, see U.S. Pat. No. 4,450,949, defined as first speed, or speed "one" machines. These so-called production-speed "two" stacking machines comprising one pair of one or more closed-ring chains spaced apart longitudinally side by side with corresponding lifting devices in a constant vertical attitude and with their top conveyor branch in motion at different levels, can in practice operate by lifting either the first product in the succession of products advanced on the first conveyor thus delaying its advance so as then take up position on the next product, or second product, in the stack to be obtained, or by lifting the second product, allowing the first one to pass by, and accelerating the movement of the chains to superimpose them.
It is precisely of the use of chains for lifting the first or second product to be stacked, particularly in stacking as described in the second case, i.e. stacking the second product on the first by accelerating the movement of the chains, that production speed limits are reached, the i.e. speed "two", which in practice cannot be exceeded by using chains.